Colorless high index glass compositions



1956 c. E. SEARIGHT ETAL 53 COLORLESS HIGH INDEX GLASS COMPOSITIONS Filed April 22', 1963 2 Sheets-Sheet 1 Frgc? AXIAL RAYS PERIPHERAL RAYS INVENTORS CHARLES E. SEARIGHT EZRA M. ALEXANDER JOHN R. RYAN BY DOMINICK LAB! Dec; 1966 c. E. SEARIGHT ETAL 3,294,553

GOLORLESS HIGH INDEX GLASS COMPOSITIONS Filed April 22, 1965 2 Sheets$het 2 l l'g b.

INVENTORS CHARLES E. SEARIGHT EZRA M. ALEXANDER JOHN R. RYAN DOMINICK LABINO ATTORNEY United States Patent COLORLESS HIGH INI JEX GLASS COMPOSITIONS Charles E. Searight, Ezra M. Alexander, and John R.

Ryan, Jackson, Miss., and Dominick Labino, Grand Rapids, Ohio, assignors to Cataphote Corporatlon,

Toledo, Ohio, a corporation of Ohio Filed Apr. 22, 1963, Ser. No. 274,523 Claims. (Cl. 10654) This invention relates to novel glass compositions particularly useful for the manufacture of glass beads but which are also suitable for the manufacture of various other materials such as flakes and the like.

Glass beads of this invention find particular utility as retroreflective lens elements which are used in highway marking paints, roadside signs, reflective sheeting, and in general are very useful in connection with any surface that requires illumination by reflection of light rays. Reflective brilliance of incident light rays is desired of retro-reflective lens elements.

Retro-reflective lens elements serve the useful purpose of focusing the light coming from a distant source to a point close to the back surface of the sphere; a reflect-or such as paint, tinfoil, and the like, should return the focused light through the beads in a direction essentia ly parallel to that of the incoming light. A retro-reflective system of this type will operate efliciently if the light is focused at, or close to, the back surface of the spherical beads, and the medium behind the beads has a high refleeting power for the focused light. It is well known that the first condition requires that the beads are made of glass having a high index of refraction.

The principles of the invention will be more particularly described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatical representation of the geometrical optics of the spherical lens;

FIG. 2 is a graph of the relative focal distance f/D of a spherical lens as a function of the effective refractive index N; and

FIGS. 3a, 3b and 3c are diagrams showing the computed trajectories of the axial, 45, and peripheral rays through spheres of effective indices of refraction N=2, 1.8, and 1.4, respectively.

The geometrical optics of a spherical lens gives the following formula for the focal distance of the axial rays (FIG. 1)

wherein 1 denotes the distance of the focus from the back surface of the sphere, r the radius of the sphere, and N=Iz/n the effective refractive index. The effective index N is the ratio of the index of the material of which the beads are made to that of the surrounding medium.

For rays passing farther away from the axis, the focal distance is shorter than that given by Formula 1. Therefore, there is no single figure that can be given as a focal distance of a sphere. Yet, for practical purposes, we want to know at what value of the effective index N the major part of the light flux is focused near to the 'back surface of the sphere. The calculation of the focal dis tances for three paraxial rays was made for a series of refractive indices between 1 and 2. The results are plotted in FIG. 1. The relative focal distance is the focal distance measured from the back surface of the sphere, expressed in terms of the diameter D of the sphere. It is'apparent that it takes an effective refractive index of 2.0 to focus the axial rays at the back surface (f/D=0), while it is enough to have N =l.8 to reach the same condition in the 45 rays and only N=l.4 with the peripheral rays. The three cases are shown explicitly in FIG.

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2. With N=2, all of the rays, with the exception of the narrow pencil of the axial rays, are focused inside of the sphere, and the concentrated light flux is spread over a small circular area over a back surface. With N=1.4, the situation is reversed; all of the rays are focused behind the sphere, with the exception of the peripheral rays. The net result, however, is roughly the same as in the first case, the strongly illuminated area being about the same size at the back surface of the sphere. The intermediate case with N =l.8 appears to give better focusing than either of the two extremes. Actually, a closer examination shows that it is' more important to have a close focal distance for the light flux between the axial and 45 rays than for the peripheral flux, because of the great loss by reflection at high angles of incidence. Consequently, an effective index between 1.8 and 2.0 is the optimum value.

If the glass beads are used in such a way that they are in contact with the light reflecting layer at their back surface only, their front surface is exposed to the air (n ='l). The refraction at the front surface is determined only by the axial refractive index n of the glass. As long as all rays are focused internally or at the back surface, the refractive index of the medium behind the beads need not be considered. It was this consideration which led to the specification of 11:19 for the desired glass composition.

There are many commercial glass compositions having the refractive index range sought for optimum brilliance, et cetera, and which are useful for the manufacture of glass beads. Heretofore. various compositions of glass have contained lead oxide, which was very desirable for many reasons, but the glass beads produced from these glasses were generally light yellow in color, and these beads would discolor further when exposed to atmospheres containing sulfide fumes due to the formation of lead sulfide. Also, the smelting of lead oxide is harmful to furnace personnel in that the fumes are hazardous.

More recently, high index, essentially colorless, leadfree glass systems have been utilized in overcoming many of the difhculties of the lead oxide containing glasses. However, these more recent lead-free glasses have been characterized by high densities, high specific heats, high cost, and relatively low surface tensions when compared to the glass compositions of the present invention. Additionally, glasses of the prior art have a tendency to become cloudy or opaque, which is known as devitrification.

Glass beads made from glass compositions of the present invention overcome many of the objectionable features of the prior art compositions. These are overcome by glass compositions which are characterized by having substantially higher surface tension and at the same time are less dense. These and other results were achieved due to the unexpected result of the addition of lime to these high barium and titanium glasses. Heretofore, it was believed that lime or calcium oxide would devitrify the glass and no substantial quantity of it could be incorporated in these high barium and titania glasses and still obtain vitreous glass beads. The calcium oxide actually lowers the melting temperature of the glass compositions, and this also is highly unexpected and not predictable since calcium oxide has a melting temperature of nearly 2600 C. but yet lowers the relatively low melting temperature of the barium oxide and titanium dioxide system significantly. Further, lime is by far the cheapest RO type material and its use in these glass compositions significantly lowers the cost of these glasses. For instance, lime is less than one cent per pound raw material cost whereas barium costs about seven cents per pound and zinc oxide costs about fourteen to sixteen cents per pound. Except for the fact that the lime forms a heretofore unknown eutectic in the glass compositions and does not devitrify in large quantity, it would not have been possible to over come the objections to the prior art compositions. Addr- The composition in mol percent of Prior Art Example 1 is as follows:

tionally, calcium oxide has a high index of refraction and I Percent assists the glass in achieving a higher index of refraction. Titanium dioxide 42.0 Basically, the glass compositions of the invention can Barium oxide 35.1 be described as titanium dioxide-silicon dioxide-barium Boric oxide 10.4 oxide calcium oxide glass systems. Silicon dioxide is per- Zinc oxide 12.4 haps the strongest, best known and most wde'ly accepted The composition in mol percent of Prior Art Example glass former of all the oxides. At least two-tenths of one 2 is as follows: percent of silicon dioxide is present in the glass composi- 7 Percent tions of the invention since it greatly assists the titanium Titanium dioxide 43.7 dioxide in the glass formation. Titanium dioxide is not Barium oxide 3&6 as well known or as widely accepted a glass forming Bone oxide oxide; and it has been found to be more difiicult to form other minor ingredients and contaminants 43 glasses with titanium dioxide in the absence of silicon dioxide, boric oxide, germanium oxide or antimony oxide. The glass compositions of the invention may be pre- Therefore, it is also advantageous to have present in the pared by melting a mix of the batch components in a glass compositions at least 0.2% by weight of boric oxide conventional glass furnace made of conventional heat and sodium oxide. Likewise, due to the fact that calcium resistant refractories free from deleterious contaminating oxide significantly lowers the melting and fining temperamaterial, particularly iron. The batch is composed of ture of these glass compositions, it is preferably present oxide or compounds decomposable to oxides under the in at least six or seven percent by weight. conditions of melting in the proper proportion to yield The glass compositions of the invention consist essenthe specified compositions. In general, titanium, alumitiolly of from about 10% to about 50% by Weight 0 num and zinc are added as oxides; barium is added in titanium dioXide, from about 10% to abo11t65% y ght whole or in part as the peroxide, the remainder as oxide of R0 yp oxides Selected from the group Consisting of or carbonate; calcium and sodium may be added as carbafillm oXido and ZiIlC oXide, at least 10% y Weight bonates; silica may be added as high grade quartz sand; ing barium oxide, from about 6% to about 40% by weight d boron may b dded as boric acid. of Calcium oXido, and from about 02% to about The glass compositions of the invention melt at temby weight of silicon dioxide. The compositions may also 30 peratures ithi th range f 1100 1400 C, The oontail} P to Y Woigbb of boric oxide, p to 16% batch mix is preferably added to the furnace continuously, y \{Vflghbof alkab metal oXldes, P to 16% y Weight or in succession increments allowing each increment to of Z111C oXlde f P to 6% y Welgbt of 31111111118; melt before the increment is added, until the glass melt Tho ranges glvon above define the glass composltlobs is brought to the desired level which may require from of the invention with indices of refraction from about 1.6 4 t 10 hQurs, 0 llt 20- After the melt has been prepared, it can be converted Table I contains various representative examples of to glass beads by conventional methods, either directly glass compositions, in parts by weight, which fall within from the melt or by pouring a stream of the molten glass the scope of the invention. into water to form cullet, particles of which are blown Table I TKO 13210 C210 No.20 SiO" B302 A1303 ZnO Refractive Density Index Table II is a comparison of two typical prior art examples and examples of the invention with respect to the more important glass bead making properties of surface or dropped through a high temperature flame or a radiant heating zone to soften them sufficiently to form spheres by the action of surface tension followed by rapid cooling to harden the spheres without devitrification.

We claim:

1. A high refractive index glass composition consisting essentially of from about 10% to about 50% by weight of titanium dioxide, from about 10% to about 65% by weight of RO type oxides selected from the group consisting of barium oxide and zinc oxide, at least 10% by weight being barium oxide, from about 6% to about 40% by weight of calcium oxide, and from about 0.2% to about 30% by weight of silicon dioxide.

2. A high refractive glass composition as defined in claim 1 containing up to 14% by weight of boric oxide. 3. A high refractive glass composition as defined in claim 1 containing up to 16% by weight of sodium oxide. 4. A high refractive glass composition as defined in claim 1 containing up to 16% by Weight of-zinc oxide. 5. A high refractive glass composition as defined in claim 1 containing up to 6% by Weight of alumina.

6. A high refractive index glass composition consisting essentially by weight:

Percent TiO 20 BaO 20 Na O 8 Si0 25 B 0 9.5 ZnO 10.5

7. A high refractive index glass composition consisting essentially by weight:

8. A high refractive index glass composition consisting essentially by weight:

Percent TiO 25 BaO 25 CaO 30 4 S10 15 B203 0.5 A1303 0.5

9. A high refractive index glass composition consisting essentially by weight:

10. A high refractive index glass composition consisting essentially by weight:

Percent T i0 B210 37.8 CaO 7 Na O 0 2 SiO 5 B 0 1 ZnO 4 References Cited by the Examiner UNITED STATES PATENTS 2,939,797 6/1960 Rindone 106-47 3,193,401 7/1965 Alexander et al. 106-54 FOREIGN PATENTS 719,067 11/1931 France.

HELEN M. McCARTI-IY, Primary Examiner. 

1. A HIGH REFRACTIVE INDEX GLASS COMPOSITION CONSISTING ESSENTIALLY OF FROM ABOUT 10% TO ABOUT 50% BY WEIGHT OF TITANIUM DIOXIDE, FROM ABOUT 10% TO ABOUT 65% BY WEIGHT OF RO TYPE OXIDES SELECTED FROM THE GROUP CONSISTING OF BARIUM OXIDE AND ZINC OXIDE, AT LEAST 10% BY WEIGHT BEING BARIUM OXIDE, FROM ABOUT 6% TO ABOUT 40% BY WEIGHT OF CALCIUM OXIDE, AND FROM ABOUT 0.2% TO ABOUT 30% BY WEIGHT OF SILICON DIOXIDE. 